It means that you don't have enough information. To get any meaning, you also need to know the unit of time. It is like saying that an object's speed is three feet. Without a time unit, it could be anything. Three feet per second, or three feet per year.

The same is true with Sieverts. You can have an exposure of 1000 millisieverts per hour, 1000 millisieverts per year, or 1000 millisieverts per century, but if you don't have a time unit, it means nothing. Remember that, and you will be amazed at how many articles about nuclear power plants get it wrong.